Can you remember a time when your most longed for birthday present was a colouring book and a set of felt-tipped pens? What age were you? Six…? 10…? Or perhaps 42?

If your answer was the last one then don't worry, you're not alone. The market for adult colouring books is booming. Last week, four of Amazon's top 10 'most wished for' titles were beautifully illustrated colouring books for grown-ups. The Secret Garden by British artist, Johanna Basford has sold more than 1.5 million copies. There's even a Game of Thrones title in the pipeline.

But it's not just keeping between the lines that has got grown-ups hooked. The whole world of child's play is experiencing a renaissance among work-weary adults.

Soft-play centre, The Play Factory in Thornaby, Teesside recorded a 'phenomenal' response when it opened up its slides and brightly coloured jungle gyms to adults-only events last April. An adult ballpit, installed in a London gallery earlier in the year, promptly sold out, and every time a city sets up a slippery waterslide, they're inundated with requests from business workers desperate to strip off their suits and slide with abandon. In the US, there have even been adult playgrounds popping up, like the Lawn on D Street in Boston, where mums and dads can play on giant fluorescent swings while the kids wait their turn instead.

So just what is it that has us so keen on kids' activities all of a sudden?

Well, first of all, it's the fact that child's play puts us into a completely different mental space compared to 'grown-up' games.

"Adult play like poker or games of squash are great fun, but they are not about free expression nor are they particularly emotive," explains Cork-based psychologist Sally O'Reilly.

"They are more about meeting other needs like power or anaesthesia - by which I mean we can medicate ourselves by gambling, wining and risking - but the child-like play isn't about medicating or dulling feelings, it's about expressing them. Which ultimately feels better."

Several times I have found myself happily creating towers with my baby son's brightly coloured foam blocks, long after he has gone to bed. Playing like a child is soothing, it's about letting go and totally immersing yourself in the task at hand. To use the buzz-word of the moment, it's mindfulness, living in the moment, enjoying it and not worrying about anything else beyond the next block, piece of the puzzle, or which felt-tip to use.

"I'm delighted about the current craze for colouring in," says Sally. "It helps people to wind down, it's contemplative, evocative of childhood and almost meditative."

She used art tools for expression and therapy when she worked with bereaved children at Barnardos, but found she missed working with art herself when she left the job.

"I've since invested in markers, crayons and sketch pads, which I have at home and in my office," she says. "Regardless of whether or not you're 'artistic', engaging with colour and images can be enormously therapeutic."

Dr John Hillery, consultant psychiatrist and director of communication and public education of the College of Psychiatrists of Ireland, agrees we all need a break from multitasking and information overload.

"Losing oneself in an activity and finding what is called 'flow', that is total absorption, has a positive effect on well-being," he says.

"In the short-term, such activities usually lead one to forget everyday worries. They also reinforce self-esteem as one gains mastery of a skill giving long-term benefits to mental health."

The importance of play for children is so great that it's been recognised by the UN High Commission for Human Rights as a basic right. Countless studies have highlighted the importance of play in developing physical, emotional, cognitive, imaginative, creative and intuitive skills and it's been observed that in fMRI scans, nothing lights up the brain like play.

And yet, when we stop being a child, we are encouraged to put away our childish things and grow up - playtime is over. The goal-orientated nature of the adult world means play is often viewed as frivolous since it comes with no obvious achievement. We know that working through lunch will get a project finished, keeping the boss happy and our job secure, but the same can't be said for spending that hour on the swings… or can it?

Taking a time out might be more productive than it looks, as it's often been observed, Archimedes shouted 'Eureka!' in the bath, not the office. Newton was relaxing in the garden when he came up with the theory of gravity.

"Exploring our creativity may help us come up with solutions to problems through the play itself or merely through giving ourselves time off from work to play," explains Sally.

"As we get older, we tend to devalue play because we're trained to believe we've learned all the lessons play can teach us and our preferences for 'play' become more sophisticated and complex.

"But we would do well to play more for playing's sake - not to have power needs, or financial control needs met - but rather to meet our need for fun and understanding how intrinsically fun is linked in with our sense of well being and happiness.

"We need to stop using words like 'childish' and 'childlike' as if they are bad things."

"We should all be playing," agrees Dr Hillery. "Particularly people who have mood or anxiety problems. Physical activity has been shown to reduce mild depression, improve coordination and cognition and allows us to discharge negative energy in a socially acceptable way. It can be engrossing with no downside if we get it wrong and it can elicit memories of times when someone else did our worrying for us."

As adults, we are acutely aware of the dangers of the world we live in, from rising house prices and job cuts to global terrorism and environmental change. With the negative background noise of the 24-hour news cycle, is it any wonder we're nostalgic for our carefree childhoods?

Dr Hillery reckons it's this desire to cast the weight of the world aside that's fuelling the current demand for simple fun.

"Our daily lives are lived at such frenetic speed and we're bombarded with demanding and worrying information constantly," he says.

"It seems to me that we are being told that we have no control over the world, something that was less the case in generations gone by. Activities that occupy us totally allow us to escape these pressures and gain a bit of control of our lives."

Of course, there has to be a happy medium. If we're missing work to play on the swings or shunning social engagements on a regular basis to colour-in, then that's cause for concern.

"As with anything we do in life it depends on the effect it has on ourselves and those around us," says Dr Hillery.

"If play becomes the overriding activity to the exclusion of work and relationships, and if we use it to avoid all adult responsibility then it is not healthy. But escapism in moderation, and in the right context, is good for us and those around us."

"To use the example of your son's bricks, as long as your play doesn't mean you're ignoring him, missing work, or letting the house fall down around you, then it's positive.

"He'll have a relaxed mum and one who can relate to the same activities as him - so play on happily!

'Colouring gives my mind time to relax...'

Model Roz Purcell (24) likes to keep a colouring book in her handbag and reckons it's improved her work. The former Miss Universe explains why:

"I'd heard about the trend for adult colouring books, but it wasn't until a friend recommended them as a great de-stressor that I got involved.

"I bought the Secret Garden and Animal Kingdom - which I'd really recommend for beginners - but I love that there are so many books out there, there's something for everyone. A lot of the themes are really therapeutic and mandalas, often a reoccurring pattern, are very calming on the mind.

"There's also something very relaxing about how 'in the moment' you are when colouring. When I started, it was just for 15 minutes here and there, but now I find I'm making time to do it.

"My focus for work is so much better. When you're sitting scrolling on the phone or watching TV, you think you're relaxing but all those images are still being processed by the brain when you go to bed.

"Colouring quiets the mind, improves your concentration, works your brain and your hands and promotes creativity. By giving my mind time to just relax, I find I'm more focused when I'm at work. Given that there can be a lot of waiting in my job, it's also really handy to keep a book in my bag.

"I've always been into mindfulness and baking used to be my way to unwind. But, although I still really enjoy it, since it's part of work for me [Roz recently released her first cookbook, Natural Born Feeder] it's not as relaxing! Colouring-in is something that's totally separate from work.

"That's not to say you can't get quite serious about it. I'm really into my colouring pencils and I've started doing my own designs and colouring them in. I did art for Leaving Cert and it was my best subject, so there's a bit of nostalgia there too.

"I know there are bound to people that might make jokes about 'models and colouring-in books' but those are the sort of people that are sitting trolling on social media. At least I'm using my brain!"